MV design guide - Schneider Electric
MV design guide - Schneider Electric
MV design guide - Schneider Electric
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Design rules<br />
Dielectric withstand<br />
The shape of parts<br />
This plays a key role in switchgear dielectric withstand.<br />
It is essential to eliminate any "peak" effect which would have a disastrous<br />
effect on the impulse wave withstand in particular and on the surface<br />
ageing of insulators:<br />
Air ionization Ozone production Breakdown of moulded insulator surface skin<br />
Distance between parts<br />
Ambient air between live parts<br />
c For installations in which, for various reasons, we cannot test under<br />
impulse conditions, the table in publication IEC 71-2 gives, according to<br />
the rated lightning impulse withstand voltage, the minimum distances to<br />
comply with in air either phase to earth or phase to phase.<br />
V 0<br />
d<br />
c These distances guarantee correct withstand for unfavourable<br />
configurations: altitude < 1 000 m.<br />
c Distances in air* between conductive parts that are live and structures<br />
which are earthed giving a specified impulse withstand voltage under dry<br />
conditions:<br />
U<br />
Rated lightning<br />
impulse withstand<br />
voltage<br />
Up (kV)<br />
40 60<br />
60 90<br />
75 120<br />
95 160<br />
125 220<br />
Minimum distance<br />
in air phase<br />
to earth and phase<br />
to phase<br />
d (mm)<br />
The values for distances in air given in the table above are minimum<br />
values determined by considering dielectric properties, they do not<br />
include any increase which could be required to take account of <strong>design</strong><br />
tolerances, short circuit effects, wind effects, operator safety, etc.<br />
*These indications are relative to a distance through a single air gap, without taking account of<br />
the breakdown voltage by tracking across the surfaces, related to pollution problems.<br />
<strong>Schneider</strong> <strong>Electric</strong><br />
Merlin Gerin <strong>MV</strong> <strong>design</strong> <strong>guide</strong><br />
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